1. Field of the Invention
The present invention relates to a signal processing method and device for transmission of such signals on a coaxial cable.
2. Description of the Related Art
FIG. 1 schematically shows a device for receiving and processing radio-transmitted modulated analog signals, for example, signals corresponding to video images. This architecture comprises a receive unit 10 which receives, via a receive antenna, a modulated analog signal corresponding to a radio-transmitted electromagnetic wave. Receive unit 10 comprises a low noise block LNB 11 which performs a pre-processing on the received modulated signal. The preprocessed modulated signal is transmitted to a signal-processing unit 12 via a coaxial cable 14. Processing unit 12 comprises a demodulation block DEMOD 15 which extracts a “wanted” modulated signal from the modulated signal transmitted on coaxial cable 14 and demodulates the extracted “wanted” signal. The demodulated “wanted” signal may, for example, be used to display video images on a television screen. In this case, receive unit 10 is located at the level of the receive antenna and processing unit 12, on which the viewer must be able to act, is located at the level of the television set.
FIG. 2 shows a simplified example of the spectrum of a modulated analog or digital signal received by block LNB 11. The signal is comprised of an initial frequency band IBW which extends, for example, between 10.7 GHz and 12.75 GHz, which corresponds to a frequency band generally used for transmission of signals between a satellite and a receive station on ground.
Initial frequency band IBW is divided into frequency channels. The frequency band width ΔB of each channel may vary, for example, from 24 MHz to 36 MHz, four channels CH1 to CH4 being shown as an example in FIG. 2. The initial modulated signal received by the receive unit corresponds to the sum of modulated “wanted” signals. The frequency band of each modulated “wanted” signal is in one of frequency channels CH1 to CH4. Frequency channels CH1 to CH4 associated with “wanted” signals of same biasing are distinct. However, the initial modulated signal may correspond to the sum of signals having different biasings. The biasing may be, for example, rectilinear (horizontal or vertical), or circular (right or left). Since different biasing waves can easily be distinct in a same signal, it is possible to transmit in a same initial signal “wanted” signals of different biasing associated with overlapping frequency channels.
The main functions of block LNB 11 are the following:                amplifying the received signals with the smallest possible noise factor, typically on the order of one decibel;        selecting the signal biasing, for example, horizontal or vertical;        converting the received signals from initial frequency band IBW into a frequency band, called the transmission band, adapted to the passband of coaxial cable 14 and to the frequency band of processing unit 12 (typically, between 950 MHz and 2,150 MHz), and        limiting the signals to be transmitted to a given frequency band.        
FIG. 3 schematically shows an example of a conventional architecture of block LNB 11. Block LNB 11 is connected to an antenna 16 receiving the initial radio-transmitted signal. Antenna 16 has two focal points 17, 18. Each focal point 17, 18, captures an initial signal of determined biasing in initial frequency band IBW. For example, focal point 18 may capture a horizontally-biased initial signal and focal point 17 may capture a vertically biased initial signal. Each focal point 17, 18, is connected via an amplifier 20, 21 to a first IRF filter 22 (image rejection filter). Amplifiers 20, 21, may be low-noise field-effect transistors. A control unit 23 drives amplifiers 20, 21 to select one of the two initial signals with a determined biasing.
The initial signal with a determined biasing is then transmitted to a mixer 25 which mixes the initial signal with a mixing frequency fM, transmitted by a frequency synthesizer 26, to displace initial frequency band IBW from the initial signal to the transmission band. IRF filter 22 has the function of avoiding for spurious signals to disturb the signal obtained by the passing through mixer 25. Frequency fM is a function of initial frequency band IBW and of the transmission band.
The signal thus mixed is transmitted to a band-pass filter BPF 27 which limits the frequency band of the signal to be transmitted to minimize the amount of power on coaxial cable 14. The mixed and filtered signal is finally amplified by an amplifier 28 before being transmitted on coaxial cable 14.
The width of initial frequency band IBW may be larger than the transmission band. In this case, frequency synthesizer 26 may generate several mixing frequencies. Each mixing frequency is adapted so that, in the mixing operation, a specific portion of initial frequency band IBW, of a width similar to the width of the transmission band, is shifted towards the transmission band frequencies. The choice of the mixing frequency is then determined by control unit 23 according to control signals generated by processing unit 12 and transmitted to block LNB 11 by coaxial cable 14.
A processing unit may comprise several demodulation blocks to process in parallel several “wanted” signals of same biasing which are extracted from the signal transmitted by the coaxial cable.
However, it may be desirable for a processing unit to be able to process in parallel several “wanted” signals of different biasing. Similarly, it may be desirable for a processing unit to be able to process in parallel several “wanted” signals coming from an initial signal having a wider initial frequency band than the transmission band, the “wanted” signals originating from different initial frequency band portions. Finally, it may be desirable for a processing unit to be able to process in parallel several “wanted” signals coming from initial signals having distinct initial frequency bands, in particular, in the case of a so-called multiple-orbit antenna which comprises several focal points oriented to receive radio signals of distinct initial frequency bands.
The device of FIG. 1 is then no longer adapted. Indeed, at a same time, the signal transmitted on coaxial cable 14 can only come from a single initial signal and only have a single determined position. Further, when the initial frequency band is wider than the transmission band, the signal transmitted on coaxial cable 14 can only correspond to a single portion of the initial frequency band.
It is then necessary to provide several blocks LNB and to connect each of them to processing unit 12 with a dedicated coaxial cable. Each block LNB then transmits, on the associated coaxial cable, a signal of a given biasing, and which corresponds to a signal of determined initial frequency band (and possibly to a given portion of the frequency band of the initial signal).
The multiplication of coaxial cables has a high cost given that to each coaxial cable correspond at least four connectors, and that processing unit 12 must comprise switches associated with each cable. Further, the cable installation and servicing have a significant cost.